1. Field of the Invention
The present invention relates to a method of manufacturing a titanium magnetic disk substrate applied to a magnetic disk for high-density recording/reproduction.
2. Description of the Related Art
In recent years, as capacity and recording density of magnetic disks have been increased, it has been attempted to decrease the thickness of the magnetic film formed on a magnetic disk substrate and the space between a magnetic head and the magnetic film.
Recording/reproduction of information on such a magnetic disk is generally performed by a CSS (Contact Start Stop) system. In this system, a magnetic head and a magnetic film surface on disk are brought into contact with each other when an operation is started, and the magnetic disk is rotated at a predetermined rate to float the magnetic head from the magnetic film surface, thereby performing recording/reproduction. In this CSS system, however, the magnetic film surface is readily placed in a contact friction state when an operation is started or ended. Therefore, improvement in CSS characteristics in a high-density recording magnetic disk is expected to be increasingly important in the future.
Under the circumstances, a magnetic disk substrate currently must satisfy both of the following two conditions.
(1) A magnetic disk substrate must have a small surface roughness (R.sub.max &lt;0.1 .mu.m).
(2) A magnetic disk substrate must have a high hardness and good CSS characteristics.
Although a aluminum alloy has been conventionally uses as a magnetic disk substrate material, it has been recently attempted to use titanium in place of the aluminum alloy as a magnetic disk substrate. This is because titanium is superior to the aluminum alloy in terms of heat resistance to enable formation of a magnetic film over a wide area by sputtering and can improve the characteristics of the magnetic film. However, the hardness of titanium is low (the hardness H.sub.v of a titanium magnetic disk substrate is at most 160) i.e., much lower than that of a conventional NiP-plated aluminum alloy (H.sub.v =about 500), resulting in poor CSS characteristics. Improvement of the CSS characteristics by increasing the hardness, therefore, is one key to applying titanium to a magnetic disk substrate.
For this reason, various methods have been attempted in order to increase the hardness of titanium. For example, Published Unexamined Japanese Patent application No. 1-112521 proposes a method of improving the CSS characteristics by using a titanium alloy having a section hardness H.sub.v of 250 or more, and Published Unexamined Japanese Patent Application No. 52-105804 proposes a method of oxidizing or nitriding a surface layer (0.03 to 10 .mu.m) after a mirror surface polishing is performed, thereby increasing the hardness of the surface layer.
The method disclosed in Published Unexamined Japanese Patent application No. 1-112521, however, is disadvantageous especially in terms of manufacturing cost. That is, of the titanium alloys disclosed as usable in this method, both the .lambda.-type and a +.beta.-type titanium alloys have low cold workabilities. In order to obtain a cold-rolled thin plate as a stock material for a magnetic disk substrate, therefore, since a plurality of cold rolling-intermediate annealing steps must be performed, manufacturing cost is significantly increased as compared with that of pure titanium. Although a .beta.-type titanium alloy of the alloys disclosed in the method has good cold workability, a large amount of alloy elements must be added. As a result, cost of the raw material to be melted is significantly increased as compared with that of pure titanium.
In the method disclosed in Published Unexamined Japanese Patent application No. 52-105804, the thickness of any obtained hardened layer is at most 10 .mu.m, and this thickness is insufficient to obtain satisfactory characteristics by the CSS system.
In addition Published Unexamined Japanese Patent application No. 57-50324 proposes a method of increasing the surface hardness of an aluminum alloy magnetic disk substrate, thereby improving the surface roughness obtained after polishing. In this method, a titanium nitride or oxide layer is formed on a polished aluminum alloy magnetic disk substrate by reactive sputtering or the like, and mechanopolishing is performed to obtain a mirror surface. When this method is used or the method is applied to a titanium magnetic disk, however, peeling of the magnetic film causes degradation of the CSS characteristics for the following reason. That is, atoms on the surface of the magnetic film fall off to inevitably form defects during the titanium nitride or oxide sputtering step, and contaminants are sealed in these defects during a polishing stop which is performed after the sputtering, thereby decreasing the adhesion strength of the magnetic film.